Procedure:
Using your finger, push on the water filled container
at various depths. Be careful not to poke your finger through!
At greater depths, the water pressure is greater, which makes
it harder to press your finger into the flexible container.

Explanation:
The deeper you go into a pool, or lake, or ocean the greater the pressure.
The pressure is due to all the water above you, pressing down
on you. If you stay at shallow depths, there's not so
much water over you so the pressure isn't very high. Go
deep enough and all the water above you can press down hard
enough to crush a submarine.